EFI conversion - return line backpressure

[turbo camino]

On a lark, while my car is apart I rigged up a test. Aeromotive 340lph pump in a bucket, hose to a pressure gauge at the return pipe inlet, loop of hose at the back of the car from return line to supply line (tank/sending unit not in the loop at this time), then hose from supply line up front back into the bucket. Ran the pump and, holy cow!, gauge shows 35psi.

If the EFI regulator is set to 58psi, does that 35psi backpressure in the return make any difference?

[Dave Koehler]

ASk yourself why there is that much back pressure.

[turbo camino]

I know why. Giant pump and tiny little 5/16 steel return line snaking its way along the entire length of the car.

Obviously I know that less would be better, but will a 58psi system operate as intended with that kind of backpressure?

And, even if I sprung for replacing the stock lines with -10 feed/-8 return, I'm still stuck with the return line going into a 5/16 pipe in the stock sending unit. How much improvement would even be possible given that limitation?

[Dave Koehler]

Ok, As long as you see the pressure you want to the injectors then yes it will work.
Only alternative at this point is a smaller gph pump.

[turbo camino]

Thank you. I know I will still have to ensure the built-in regulator (Sniper EFI, I'm poor and they're cheap) has a large enough seat to handle the volume, an bigger external regulator may be necessary once I get to that point in the project. I have heard the Holley uses a reg very similar to OEM stuff with a very small orifice and I wouldn't be surprised if I find it can't handle a big pump.

Right now the car's going back together after a long down time, with a Qjet, mechanical pump, & carb tank sending unit. But I'm accumulating bits and pieces like the pump, EFI sending unit as I can. While the tank is still out and none of the plumbing is attached at either end of the car I figured it would be a good time to poke around at things and make a plan.

[2seater]

I know why. Giant pump and tiny little 5/16 steel return line snaking its way along the entire length of the car.

Obviously I know that less would be better, but will a 58psi system operate as intended with that kind of backpressure?

And, even if I sprung for replacing the stock lines with -10 feed/-8 return, I'm still stuck with the return line going into a 5/16 pipe in the stock sending unit. How much improvement would even be possible given that limitation?

The easiest thing to do is try a larger return with as short a piece of 5/16" line at the end as you think will be possible. Eliminating the longest part of the small run will definitely lower the backpressure and although not a perfect solution, it should help. Even moving up to a #6 hose and reduced close to the tank is a substantial change.

[turbo camino]

It probably makes the most sense to run a new supply line and use the original 3/8 supply as the return. Original 5/16 return line can be repurposed as a tank vent. In fact, this being a carbureted car I think it was originally used as a vent/vapor line.

So, what's the most economical way to replace the OEM hard line? Goodridge 910 series PTFE is freakin' $20 per foot. 25ft of 3/8 nickel-copper tube is $90. Gates Barricade MPFI hose is something like $60/25ft, not crazy about using plain rubber hose for a run that long (am I being paranoid?). Aluminum tubing is cheap but fragile...

Reproduction stainless 3/8 line is available but is $150 plus what I'm assuming would be $50+ shipping. Blech.

This is a junky-appearing stealthy street car, bench seat, column shifter, doesn't even have a tach, so race track tech inspections aren't an issue but obviously I'd much prefer to not burn to death if at all possible.

[77cruiser]

The instructions I got with my Aeromotive regulator said to use a larger return than pressure side 3/8 feed & 1/2 return I using it for a carbed deal with 320lph in tank.

https://www.summitracing.com/parts/aei- ... plications

[mk e]

Return lines are at little to no pressure so its pretty common to use plain only rubber fuel hose which is pretty cheap.

Edit: forgot the most important part.....since you are running a steel line that can handle pressure you really don't have a any problem as along the the return pressure is under the 58 set point as that means the regulator is still in control. Worst case is either engine off (using no fuel but voltage to pump about 12V) or at idle (engine using a little fuel but pump voltage about 14V so its pumping more)....so just check your fuel pressure at those 2 points and if its 58psi your return is fine. The return line pressure will heat the fuel a bit more I guess butthat is about the only concern.

What ECU are you planning to use? I ask because you have 2 more options depending.

1 you could put the pump n a solid state reply and have the ECU pwm the signal so you have the flow you need but not a lot more so no issue with pressure drop in the return line and less fuel heating. I do this with the aeromotive A1000 I use in the frankenferrari....I have a small table (4x4 iirc) that is fuel mass flow on the x, Vbat on the y, PWM% in the cells. To set it up I drop the PWM number until I see the fuel pressure start to drop thten add 10% and call it done. I have a 5/16 rubber return hose about 4 ft long and the regulator has no problem maintaining 58psi to the rail.

2) you can add a fuel pressure transducer so the the fuel pulse is compensated for measured fuel pressure and stop worrying about maintaining a perfect 58pi (or what even you tuned at). I have this setup and even with the car running out of fuel (look at the the cyan fuel pressure trace bouncing between about 30 and 0) the lambda wasn't to awful bad....I was just starting tuning so didn't even realize the fuel issue until it stalled and wouldn't restart. The point is with a setup like this it doesn't matter even a little bit if the fuel pressure creeps up or down a few psi....10s or psi can be a problem but a few psi is not noticeable.

[turbo camino]

If I ever get around to it it'll be Holley Sniper throttle body thing, fairly limited. I'm not sure if it supports/can be tricked into supporting variable pump speed. Probably not though it does have the option of using the 'full' (whatever that means) Holley tuning software via laptop which unlocks things not available just thru the handheld, like boost enrichment and some other stuff. I haven't dug into it really.

I have built a homebrew PWM controller for a carb application/A1000 pump years ago using an Arduino (LOL, right?). I'm not an engineer but I could hack one together again if I had to.

[Kenova]

1 you could put the pump n a solid state reply and have the ECU pwm the signal so you have the flow you need but not a lot more so no issue with pressure drop in the return line and less fuel heating. I do this with the aeromotive A1000 I use in the frankenferrari....

Do you have a capacitor between the SS relay and the pump? Would it make any difference in reliability or how the pump runs?

Ken

[mk e]

Do you have a capacitor between the SS relay and the pump? Would it make any difference in reliability or how the pump runs?

Ken

I don't have a cap in there....never really thought about it but it does seems to make some sense. Normally when there is fuel pressure is very stable. I have the filtering on the fuel pressure basically tuned off so there is a couple psi of noise (the gauge on the regulator is dead stable) but not at or even near the pwm frequency (10khz) the best I can tell and is there with the pwm function disabled so I've assume it is just normal system noise

On the throttle I did add a cap after the driver I was using failed. The driver maker recommended adding a large capacitor beyond the small one they supply but that is an h-bridge not a simple relay that is rated for switching inductive loads that I use for the pump.